We propose to map and characterize by ultrastructural and immunological means the surfaces of cells, particularly the surface immunoglobulins of some lymphocytes. The techniques to be employed include radioautography, labeling of antigen with electron opaque markers, freeze etching and replication of suitably labeled cells. The labels will be directed against specific components of the cell surface as determined by immunological assays. With these approaches we hope to learn the topographical distribution of surface markers among different classes of lymphocytes and to relate these findings to the immunological function of these cells.